Powder coating compositions are dry free flowing powders applied in the fusion coating process. A fusion coating process is a process in which a coating powder is distributed over a substrate and, when heated fuses to form a continuous film. The substrate may be heated or unheated when the powder is applied thereto. Heat supplied from the substrate or from an external source, such as an oven, causes the powder to fuse into the continuous film. Known fusion coatings processes for application of powder coating compositions to a substrate include electrostatic spraying, fluidized bed coating and hot flocking.
Polyurethanes, hybrid polyester and epoxy resin combinations, TGIC (triglycidylisocyanurate), Primid resin systems using polyisocyanate curatives and non-TGIC type platform finish coatings are fused to form a film which has a uniformly random series of irregularities or defects, ranging from a slight to severe orange peel surface which are in some cases unique non-repetitive random patterns.
Powder coating compositions may be made of many different resin materials including epoxy, epoxy-polyester, urethane-polyester, TGIC-free polyesters and acrylic coating materials. TGIC-free refers to resins which are free of triglycidylisocyanurate. Other components or constituents include curatives, flow aids, degassing agents, catalysts, pigments, modifiers, fillers and charge inhibitors.
One problem facing applicators of powder coating materials involves formation in the film of a type of surface defect. In the industry surface defects know as orange peel or surface tension are known. Surface tension can be defined as an irregularity which affects the flow resulting in orange peel after product solidification. However for some applicators this can be controlled by increasing or decreasing film build or thickness. The presence of such anomalies of surface tension in the film may result in rejection of the coated article by the customer. Any such rejection may result in discarding of many thousands of powder coated articles or in costly reworking such articles.
Typical efforts to minimize surface tension have been directed toward improving powder coating material flow characteristics. Efforts such as increasing extruder temperature and mix times and increasing flow additives have been tried. These efforts have not been satisfactory because they may result in undesired side effects such as a condition known as “edge pulling” Edge pulling is a condition in which the coating pulls away from the corners of the coated substrate resulting in incomplete formation of the finish.